The present invention relates to a trunk interface for the Integrated Services Digital Network (ISDN) system which uses the North American Standards (which will be referred to as the T1, hereinafter) system together with the Conference of European Post and Telecommunications Administrations (which will be referred to as the CEPT, hereinafter) system and, in particular, to a trunk interface which is equipped to an ISDN Private Automatic Branch Exchange (which will be referred to as the PABX, hereinafter), a Network Termination 12 (which will be referred to as the NT12, hereinafter) or similar apparatus for interfacing with the official exchange which provides ISDN services or the other PABX at the primary rate through trunk lines.
According to recommendations of the CCITT, there are two kinds of systems in the primary rate ; one is CEPT system having the transmission rate of 2.048 Mbps and the other is T1 system having the transmission rate of 1.544 Mbps. In the CEPT system, one multiframe consists of 16 frames of which each frame consists of 32 channels and the channel No.16 of each frame except 0th frame is used a signalling channel and the others are used as B or D channels (30B+1D). On the other hand, in the T1 system, one multiframe may consist of either 12 frames or 24 frames of which each frame consists of 1 framing bit(F) and 24 channel (23B+lD). As the above-mentioned, the two systems are different from each other in aspects of bit rate and frame alignment formation. And clocks offered to the respective system are also different from each other.
For these reasons, a conventional trunk interface for the primary rate is usually constructed in order that it supports only one system of the CEPT or the T1.
Moreover, the most part of he conventional trunk interface separates receiving informations of 30B+1D (CEPT) or 23D+1D (T1) into B channel informations and D channel informations, and it transmits the B channel informations to a switch matrix of the system which has the trunk interface and the D channel information to a high order level of the system which has the trunk interface. Also the conventional trunk interface joins the B channel informations from the switch matrix and the D channel informations from the high order level together, and it transmits the informations of 30B+1D (CEPT) or 23B+1D (T1) to the other exchange.
Accordingly, the conventional trunk interface must utilize a microprocessor which has serial channels such as High Level Data Link Control / Synchronous Data Link Control (HDLC/SDLC) in order to transmit the D channel informations extracted by a D channel controller to the high order level and receive the D channel informations from the high order level, otherwise it must utilize a special communication module which is controlled by microprocessor in order to communicate with the high order level of the system.
Therefore, the trunk interface of the prior art has several problems, that is to say, the selection of the microprocessor is restricted within narrow limits, the cost of production comes high because the trunk interface has to be endued with a microprocessor having the special functions or additionally a special communication module, and the load of the microprocessor is increased because the microprocessor directly take part in the control of D channel communication.